1. Field of The Invention
The present invention relates to a high density ethylene polymer and a method for producing the same. More particularly, the present invention is concerned with a high density ethylene polymer which is advantageous in that it has not only excellent mechanical properties, such as high impact resistance and high stiffness, but also excellent moldability, such as high melt flowability, and a method for producing the same.
2. Prior Art
High density ethylene polymers have been put into a wide variety of practical uses in the form of molded articles produced by various molding methods. For example, as a representative method for obtaining a molded film, there can be mentioned an inflation method in which a high density ethylene polymer is melted and the resultant molten polymer is extruded through a die while blowing air into the molten polymer being extruded, to thereby inflate the molten polymer extrudate. Further, as examples of methods for producing molded articles having desired shapes, there can be mentioned a blow molding method in which a high density ethylene polymer in the molten state is introduced into the cavity of a mold to form a molten polymer mass in the cavity. Air is blown into the molten polymer mass in the mold cavity, thereby causing the molten polymer mass to be expanded and pressed onto the inner wall of the cavity. Also an injection molding method is known in which a high density ethylene polymer in the molten state is injected under pressure into the cavity of a mold to fill the cavity.
To all of these various molding methods for high density ethylene polymers, it is common that a melt-molding operation be performed in which a high density ethylene polymer is melted by heating and the resultant molten polymer is molded. Therefore, the behavior of the high density ethylene polymer at heat-melting, i.e., melt properties, are extremely important in molding the high density ethylene polymer.
Especially in the injection molding method, melt properties, especially melt flowability, of a high density ethylene polymer are crucially important for achieving a satisfactory molding.
In the present invention, the term "melt flowability" means a property corresponding to the extrusion load at extrusion of a molten resin through an extruder. As examples of indices which can be used as a yardstick for melt flowability, there can be mentioned M.sub.I, H.sub.MI and M.sub.IR.
In the present invention, M.sub.I means an MFR (melt flow rate) as measured at 190.degree. C. under a load of 2.16 kg. H.sub.MI means an MFR as measured at 190.degree. C. under a load of 21.6 kg. M.sub.IR means the ratio of H.sub.MI to M.sub.I, that is, H.sub.MI /M.sub.I.
In general, with respect to each of M.sub.I, H.sub.MI and M.sub.IR, the larger the value, the higher the melt flowability.
However, in practice, the desired properties of a polymer as a molding material vary depending on the molding method employed. Therefore, which index can be suitably used as a yardstick for melt flowability varies depending on the molding method employed. For example, in the injection molding method, for surely obtaining a molded product having high impact resistance, there is a tendency to use a high density ethylene polymer having a narrow molecular weight distribution. In general, a high density ethylene polymer having a narrow molecular weight distribution has a small M.sub.IR, so that, as an index for melt flowability, M.sub.I and H.sub.MI are mainly used.
Conventionally, a high density ethylene polymer for use in the injection molding method has generally been produced by a polymerization using a Ziegler-Natta catalyst, which contains titanium, or using a chromium-containing catalyst. However, in general, a high density ethylene polymer produced by using such a conventional catalyst tends to have a broad molecular weight distribution, and it has been impossible to render a molecular weight distribution narrower than a certain breadth. In order to solve this problem, it has been attempted to improve the moldability of a high density ethylene polymer to be used for injection molding by lowering the molecular weight of the polymer so that the values M.sub.I and H.sub.MI can be increased. However, this attempt has led to a disadvantage in that the produced high density ethylene polymer virtually contains extremely low molecular weight components, so that the mechanical properties, such impact resistance, of the polymer are markedly lowered. That is, when good mechanical properties are desired, M.sub.I and the like cannot be largely increased. This means that, for example, injection molding cannot be performed at a high rate.
On the other hand, it has recently been found that when a catalyst system comprising a solvent-soluble transition metal compound containing at least one halogen, such as bis(cyclopentadienyl)zirconium dichloride, and an aluminoxane is used for homopolymerization of ethylene or copolymerization of ethylene with an .alpha.-olefin, the catalyst system exhibits high polymerization activity. With respect to the details of this technique, reference can be made to, for example, Examined Japanese Patent Application Publication No. 4-12283 (corresponding to DE 3127133.2). Further, an improved technique over the technique disclosed in the above-mentioned Examined Japanese Patent Application Publication No. 4-12283 is disclosed in, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 60-35007. The catalyst system proposed in these prior art documents is attracting attention as the so-called metallocene catalyst system. By using such a metallocene catalyst system, an ethylene polymer having a narrow molecular weight distribution can be produced, wherein, when the ethylene polymer produced is an ethylene copolymer, the copolymer has not only a narrow molecular weight distribution, but also a narrow copolymerization distribution (i.e., narrow distribution with respect to the proportions of different component monomer units constituting the copolymer). By virtue of having a narrow molecular weight distribution, an ethylene polymer produced by using such a metallocene catalyst system has advantages in that it has high mechanical properties, such as high impact resistance, that it is substantially free of low molecular weight components and high molecular weight components (both of which pose problems, such as high tack and gellation), and that it has excellent properties, such as high resistance to solvent extraction and high transparency. Therefore, energetic researches have conventionally been made on the use of a metallocene catalyst system mainly for producing, for example, a linear low density ethylene polymer (LLDPE), a very low density ethylene polymer (VLDPE) and an ultralow density ethylene polymer (ULDPE). As mentioned above, on one hand, an ethylene polymer produced by using a metallocene catalyst system has such great advantages by virtue of the narrow molecular weight distribution thereof; however, on the other hand, such an ethylene polymer has a problem in that it has an extremely poor moldability due to its narrow molecular weight distribution. Because of this problem, conventionally, with respect to the development of the application of a metallocene catalyst system in production of high density ethylene polymers, which are required to have a good balance between mechanical properties and moldability, a remarkable progress has not yet been achieved.
In order to solve the above-mentioned problem, it has been proposed to broaden a molecular weight distribution by using, for example, a method in which use is made of a plurality of reactors or a method in which use is made of a plurality of types of metallocene catalysts in combination. Specifically, Unexamined Japanese Patent Application Laid-Open Specification No. 60-35008 proposes a method for rendering a broad molecular weight distribution wherein use is made of a mixture of at least two types of transition metal compounds as a catalyst. Further, a method for rendering a broad molecular weight distribution by using a customary multi-step polymerization process is known. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 3-234717 discloses a method for improving the melt properties of a polymer, wherein a multi-step polymerization process is performed by using an olefin polymerization catalyst comprising a transition metal compound and an organoaluminumoxy compound. Further, for example, in Unexamined Japanese Patent Application Laid-Open Specification Nos. 61-57638, 6-49287, 6-136195 and 6-206939, it is attempted to improve the moldability-of a polymer by taking a measure to broaden the molecular weight distribution. When these conventional methods for rendering a broad molecular weight distribution are used, the moldability is improved; however, a lowering of other properties inevitably occurs, so that it becomes impossible to obtain a high density ethylene polymer product having excellent properties, such as high mechanical properties, which are achieved by a narrow molecular weight distribution.
In Unexamined Japanese Patent Application Laid-Open Specification Nos. 7-90021 and 7-97408, with respect to the production of a film by inflation molding, it is attempted to improve moldability, such as the stability of a bubble (i.e., a molten polymer tube inflated for forming a film therefrom), by a method in which melt tension is increased by maintaining a relatively high intrinsic viscosity. However, such a method has a problem in that a lowering of melt flowability inevitably occurs, so that it becomes difficult to perform molding at a high rate.
In order to solve these problems, in the production of an ethylene copolymer by using a metallocene catalyst, it has recently been attempted to produce an ethylene copolymer which is advantageous in that it not only has both a narrow copolymerization distribution and a narrow molecular weight distribution, but also has excellent melt properties. For example, International Patent Application Publication No. W093/08221 proposes a method for producing an ethylene copolymer having improved melt flowability while maintaining a narrow molecular weight distribution thereof. In this proposed method, copolymerization is performed by using a specific metallocene catalyst to thereby cause the ethylene copolymer to have a long branched chain. However, such an ethylene copolymer has a problem in that, although the melt flowability is improved to some extent, the mechanical properties, such as impact resistance, are considerably lowered, as compared with those of an ethylene copolymer produced by using an ordinary metallocene catalyst.
As described hereinabove, the prior art techniques have problems in that it is impossible to obtain an excellent high density ethylene polymer having both excellent mechanical properties and high melt flowability.
Therefore, if an excellent high density ethylene polymer having both excellent mechanical properties and high melt flowability is obtained, the above-mentioned problems accompanying the ethylene polymer produced by using a titanium-containing Ziegler-Natta catalyst or a chromium-containing catalyst, and also the above-mentioned problems accompanying the ethylene polymer produced by using a conventional metallocene catalyst can be completely solved and the application fields of ethylene polymers can be greatly expanded. That is, such an excellent high density ethylene polymer is of extremely great commercial significance.